Field of the Invention
The invention relates to methods and apparatus for cleaning articles using supercritical and/or near-supercritical fluids. In particular, the present invention relates to using differences in contaminant solubility and solvent density at various temperatures and/or pressures to effect cleaning action, to influence solvent and/or contaminant movement in cleaning apparatus, and to facilitate concentration of contaminants within cleaning apparatus and their subsequent removal.
Cleaning Using Solvent Action It has long been known to use solvents in removing organic and inorganic contaminants from articles. In such processes, the contaminated article to be cleaned is contacted with the solvent to solubilize and remove the contaminant. In a vapor degreaser, subsequent evaporation of the solvent separates the solvent and the contaminant, and the solvent vapors are redirected to the article to further clean it. The contaminant is typically concentrated in the evaporation step, being removed as a precipitate, a separate liquid phase, or as a concentrated solution in the original solvent.
An example of the above process is described in U.S. Pat. No. 1,875,937, issued Sep. 6, 1932 to Savage. Grease is removed from the surface of metal castings and other nonabsorbent bodies by means of solvents, while contaminants collect in the bottom of the apparatus and are drawn off from time to time through a valve.
One of the drawbacks of this type of cleaning process is that the cooling surfaces also have a tendency to condense water out of the atmosphere in addition to cooling and condensing the solvent. This condensed water then becomes associated with the solvent and thus comes into contact with the metal parts of the cleaning apparatus and with the article being cleaned.
U.S. Pat. No. 2,123,439, issued Jul. 12, 1938, to Savage, describes how this problem of condensing water with the solvent may be overcome by first contacting the atmosphere with condensing surfaces at a temperature above the dew point of the atmosphere in which the operation is being carried out, but substantially below the condensing temperature of the solvent. The condensed solvent is drawn off for use in the cleaning process, while the remaining vapors are brought into contact with still cooler surfaces (cooler than the dew point) to condense out the water so it can be removed.
An alternative to the above process of condensing the solvent on a cold surface and then contacting the article to be cleaned with condensed solvent is to cool the article itself. For example, U.S. Pat. No. 3,663,293, issued May 16, 1972, to Surprenant et al., describes how the degreasing of metal parts may be accomplished by generating vapors of a solvent from a liquid sump, establishing a desired level of solvent vapor by adjusting the temperature of condensing means, and introducing a contaminated cold article into the solvent vapors, thereby causing the vapor to condense on the article. Condensate containing the contaminant falls from the article into the sump, and the article is removed from the solvent vapor when its temperature reaches the solvent vapor temperature (thus precluding further solvent condensation on the article).
Cleaning Using Supercritical Fluids
In an effort to improve on vapor degreasing methods, supercritical (and near-supercritical) fluids have been used as solvents to clean contaminants from articles. NASA Tech Brief MFS-29611 (Dec. 1990), describes the use of supercritical CO.sub.2 as an alternative for hydrocarbon solvents conventionally used for washing organic and inorganic contaminants from the surfaces of metal parts.
A typical supercritical fluid cleaning process involves contacting the part to be cleaned with a supercritical fluid. The supercritical fluid, having solubilized contaminants and thus removing them from the part, then flows to a zone of lower pressure through an expansion valve. This depressurization causes the solvent fluid's state to change from supercritical to subcritical, resulting in separation of the solute (that is, the contaminant) from the solvent. Relieved of its burden of contaminant, the cleaned solvent fluid is then compressed back to a supercritical state and again brought into contact with the part if further cleaning is desired.
A different approach to cleaning with supercritical fluids is described in U.S. Pat. No. 4,944,837, issued Jul. 31, 1990 to Nishikawa et al. The method is applied to cleaning a silicon wafer in an atmosphere of supercritical carbon dioxide which contacts the wafer to solubilize the contaminant. After cleaning is complete, carbon dioxide is cooled to below its supercritical temperature (i.e., the system pressure is reduced and the carbon dioxide attains equilibrium between the liquid and gas phases) before removal of the cleaned wafer from the apparatus.
While effective, these processes are relatively inefficient because of the energy consumed in each pressurization-depressurization cycle. Further energy losses and increases in equipment complexity are associated with moving the solvent through the apparatus in both supercritical and subcritical states.